Central traffic signal control

ABSTRACT

There is disclosed a central control station and a plurality of local stations each including a signal controller responsive to commands from the central control station for controlling the signal lights at each local station. A computer at a central control station monitors changes in traffic trends and can send a signal to a local station to control the signal controller sequence at any intersection with an appropriate offset between intersections. The system smoothly changes from local (isolated) operation to remote (computer control) operation without adversely effecting traffic flow. The central control station includes a slave driver associated with each local station. Each slave driver is connected over a two-wire telephone type line to a corresponding slave unit which in turn is coupled to the signal controller associated with that local station. In a non-actuated system with the signal controller of the local station at its rest or dwell position, the computer can issue a hold command that is transmitted by the slave driver over the two-wire line to the slave unit. This command changes control from local to remote. Thereafter, the computer may issue an advance command that moves the signal controller to its next position. The advancement of the controller is sensed by the slave unit which in turn transmits a status command to the slave driver and thence to the computer indicating that the controller did, in fact, advance. If no status command is received the intersection is dropped by the computer and returns to local operation. In a semi-actuated or fully actuated system one or more detectors are provided at the intersection. For example, one or more detectors could be used for phase A (main street), and one or more detectors for phase B (cross street). With an actuated system one or more detection responsive means may couple to one or more detectors and the signal controller for controlling the stepping of the signal controller. Local signal controller sequencing may thus be responsive to computer commands and local vehicle detections.

United States Patent Siklos et al.

Oct. 9, 1973 Primary Examiner-William C. Cooper AtmrneyWolf, Greenfield& Sacks [57] ABSTRACT There is disclosed a central control station and aplurality of local stations each including a signal controllerresponsive to commands from the central control station for controllingthe signal lights at each local station. A computer at a central controlstation monitors changes in traffic trends and can send a signal to alocal station to control the signal controller sequence at anyintersection with an appropriate offset between intersections. Thesystem smoothly changes from local (isolated) operation to remote(computer control) operation without adversely effecting traffic flow.The central control station includes a slave driver associated with eachlocal station. Each slave driver is connected over a two-wire telephonetype line to a corresponding slave unit which in turn is coupled to thesignal controller associated with that local station. In a non-actuatedsystem with the signal controller of the local station at its rest ordwell position, the computer can issue a hold command that is transmitted by the slave driver over the two-wire line to the slave unit.This command changes control from local to remote. Thereafter, thecomputer may issue an advance command that moves the signal controllerto its next position. The advancement of the controller is sensed by theslave unit which in turn transmits a status command to the slave driverand thence to the computer indicating that the controller did, in fact,advance. If no status command is received the intersection is dropped bythe computer and returns to local operation.

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TRAFFIC SENSORS f' COMPUTER F al/f SLAVE DEMOTI'E TAfiON SLAVE DRIVERSCENT R AL TA1ON COMMUNICATION LINK SLAVE UNIT DEMAND DEMAND $|GNAL IRESPOND RESPOND CONTROL- UNIT UNIT LER LIGHTS 4 25B 26 2s DETECT DETECT-OR(S) OR(S)' 0A! OBI 26A 26B DEMAND DEMAND- RESPOND RESPOND UNIT A UNIT25',

I Q DETECT- DETECT- OR(S) OR(S) INV'ENTORS GREGORY S/KLOS JAMES- B.RUDDEN WOLF; GREENF/ELD a SACKS Pmmm-um 9191s saw v13 av 5.;

d w maxi IAN CLEAR GA VEHICLE CLEAR K m L R QB GREEN PLUS WALK QBVEHICLE CLEA (58 RED CLEAR GREEN KIP SKIP .v km 5% d q Q55 1 Eu mmmwmm FF m mmwmm L 3&5 LL L INVENTORS GREGORY S/KLOS JAMES B. RUDDE N WOLF,GREENF/ELD 8 SACKS CENTRAL TRAFFIC SIGNAL CONTROL RELATED APPLICATIONFIELD OF INVENTION The present invention relates in general to a trafficcontrol system comprising a central control station and a plurality oflocal stations each including a signal controller for controlling signallights associated with a traffic intersection. A computer at the centralcontrol station is capable of transferring control between localoperation and computer controlled operation. More particularly, thepresent invention relates to a computerized traffic signal controlsystem that employs a twowire telephone-type line between the centralstation and each remote station for sending control commands to thelocal station and receiving status commands over the same two-wire linefrom the local station when the local signal controller has advanced.

BACKGROUND OF THE INVENTION In recent times the computer has played amore active role in controlling traffic movement, particularly incongested urban areas. These computer systems are generally responsiveto changes in traffic volume, traffic density and traffic flow toprovide different offset cycles and time splits for progressiveintersections to thereby provide smooth traffic flow. In a non-actuatedsystem a two-wire communication link has been used to send and receivecommands from a central station to a remote station. Usually a separatetwo-wire line is used for each such remote station. However, in asemiactuated system for an intersection including a number of lanes,additional wires have been necessary to communicate between the centralstation and the local station. This is due at least in part to the factthat a plurality of separate detections have been stored by the computerin order to effectively control each intersection.

OBJECTIVES It is one objective of the present invention to provide -atraffic control system comprising a central control station, and aplurality ofremote stations wherein a twowire telephone type lineconnects from the central station to each remote station, wherin thesystem is adapted to cause advancement of the local signal controllerand the central station is adapted to receive status signals from theremote station indicative of the advancement of the controller. Anotherobjective of the present invention is to provide a traffic controlsystem comprising a computer located at the central station and adetection responsive means coupled to each signal controller which isresponsive to detections associated with the controllers intersection tocause selective advancement of the local controller. Still anotherobjective of the invention is to provide a traffic control system inaccordance with the preceding objects wherein no more than one two-wireline couples the central station to each local station regardless of thenumber of local detectors and detection responsive means are associatedwith each local station.

THE INVENTION According to the invention, the traffic control systemcomprises a central control station and at least one local stationincluding a traffic signal controller for controlling the trafficintersection. A two-wire telephone-type transmission line intercouplesthe central control station and each local station. The central controlstation comprises circuit means for selectively impressing a firstsignal level on the two-wire line when the central station decides tocontrol the operation at the local station. This circuit means may undercomputer control selectively impress a second signal level on thetwo-wire line for a predetermined duration when the central controlstation decides to advance the signal controller at the local station.The local station also includes circuit meanswhich respond to the firstsignal level for selectively rendering the local station responsive tothe second signal level, and means thereafter responsive to the secondsignal for advancing the local signal controller. In addition, the localstation includes status means responsive to the advance of the signalcontroller for generating a status signal during the predeterminedduration of the second signal on the twowire line. The central controlstation includes status receiving means for detecting the status signal.

In accordance with another aspect ofv the invention one or more vehicledetectors are located at each local intersection to providesemi-actuated or fully-actuated operation. One or more detectionresponsive means couple to the vehicle detectors, the circuit means ofthe local station and the local signal controller, and include a vehiclememory actuable from the vehicle detector. In the following expositionthe detection responsive means is illustratively depicted by the demandrespond unit. Each detection responsive means comprises a circuit meansfor controlling the advancing of the controller coupled to the means foradvancing and the vehicle memory, and adapted to selectively inhibit theadvancing means during a rest or dwell interval of the controller whenno vehicles have been detected. A subsequent vehicle detection wouldallow the controller to advance.

In accordance with another aspect of the invention, a second embodimentof a detection responsive means is shown. This embodiment is responsiveto phase B detections, for example, when the controller is in the phaseA dwell interval to cause the controller to step. Furthermore, thedetection responsive means is responsive to continued phase Bdetections, occuring at a high enough frequency, when the controller isin the phase B extension interval to prevent the controller fromstepping out of this phase B extension interval.

A pair of detection responsive means may also be used at a localstation, one associated with the phase A detector or detectors and theother associated with the phase B detector or detectors. With this typeof arrangement the controller stays in either dwell interval awaiting adetection on an opposing phase. During an extension interval of eitherphase continued detections on the phase of interest would extend theextension interval.

The present invention also provides timing circuitry at each detectionresponsive means for detecting an extended force-off signal (this isactually an extended advance pulse in one embodiment) and forcing thecontroller out of the phase A dwell interval, for example, even thoughno detection occurred on phase B.

Numerous other objects, features and advantages of the invention shouldnow become apparent upon a reading of the following exposition of theinvention in conjunction with the following drawings in which:

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic diagram of a typicalintersection including a main street and cross street;

FIG. 2 is a block diagram of a traffic control system in accordance withthe invention for non-actuated operation;

FIGS. 3A and 3B are a circuit diagram of a slave driver;

FIG. 3C shows another arrangement for part of the circuitry of the slavedriver of FIGS. 3A and 33;

FIG. 4 is a circuit diagram of a slave unit;

FIG. 5 shows voltage waveforms on the telephone line for the local,remote, and resynchronous modes of operation;

FIG. 6 is an interval diagram showing controller signals fornon-actuated and semi-actuated operation;

FIG. 7 is a block diagram of a semi-actuated traffic control system inaccordance with this invention;

FIG. 8 is a circuit diagram of one embodiment of a demand-respond unitof FIG. 5;

FIGS. 9A and 9B are a schematic diagram of a fullyactuated trafficcontrol system in accordance with this invention;

FIG. 10 is a circuit diagram of another embodiment ofa demand-respondunit used in the block diagram of FIG. 7;

FIG. 11 is a block diagram of another embodiment of the invention; and

FIG. 12 is an interval diagram showing controller sig nals forfully-actuated operation.

EXPOSITION FIG. 1 is an illustration of a typical intersection includinga main street referred to as phase A, and a cross street referred to asphase B. Both the main street and cross street may each include one ormore lanes carrying vehicles traveling in opposite directions asindicated by the arrows. A pair of signal lights SA control vehiclemovement on phase A while signal lights SB control vehicle movement onphase B. Pedestrian signal lights (not shown) may also be associatedwith the intersection shown in FIG. 1. In a semi-actuated orfully-actuated system one or more vehicle detectors may be located inthe path of travel associated with phase A or phase B. A conventionalimbedded inductive loop vehicle detector may be used. In FIG. 1 thedetectors for phase A are detectors DAl and DA2, while the detectors forphase B are detectors DB1 and DB2.

NON-ACTUATED SYSTEM FIG. 2 is a block diagram of a non-actuated trafficcontrol system constructed in accordance with this invention. The systemgenerally includes a central station, a communication link, and aplurality of remote stations. The central station of FIG. 2 includes acomputer and a pair of slave drivers 22 each having a map display unit23 associated therewith.

The map display unit 23 may have one or more indicator lights forindicating different conditions at the associated local station, such asthe main street being green. The computer 20 may be a conventionalgeneral purpose computer programmed to control the operation of aplurality of remote signal controllers. A number of traffic sensors maybe coupled to computer 20 to provide it with data on traffic volume,traffic density, traffic speed and any changes in these parameters. Eachslave driver 22 receives commands from computer 20 when the computerdecides to control the intersection associated with that slave driver.These commands are transmitted over two-wire line 25 to slave unit 24.Slave driver 22 also receives status commands from slave unit 24 whichindicate that the signal controller has advanced. A change in polarityof the status command indicates to the computer that the intersection isabout to display a green (proceed) signal for the main street. It is atthat time that the computer may decide to control the local station. 7

The communication link may be a low-grade twowire telegraph line 25which couples between slave driver 22 and slave unit 24. FIGS. 5A and 5Bshow the voltage patterns on this two-wire line for both remote andlocal operation.

Each slave unit 24 couples to a signal controller 26 which in turncontrols signal lights 28. When intersection I, for example, is undercomputer control the slave unit 24 receives an advance command thatactuates the cam of signal controller 26 thereby advancing thecontroller to its next interval of operation. When the intersection isreturned to local operation the dial unit of the signal controllercauses actuation of the cam at fixed intervals. The specific operationof the system shown in FIG. 2 should be more clearly understood after adiscussion of the specific embodiments shown in FIGS. 3 and 4 of a slavedriver and slave unit, respectively.

An interval diagram indicating signal light sequences is shown in FIG.6. For non-actuated operation the cam signal S1 is true (marked by X)for the main street green interval. Actually, the S1 siganl appears oneposition ahead of the G1 lamp so that the status signal returned to thecomputer indicates when the main street is about to turn green. The S1signal is coupled to the slave unit which is discussed hereafter withreference to FIG. 4. For non-actuated operation the controller signalsS2-S4 shown in FIG. 6 are not used. In FIG. 6 a diagram for a 16position controller is depicted. For a 12 position controller positions12-15 are omitted.

SLAVE DRIVER A circuit implementation for a slave driver is shown inFIGS. 3A and 33. All relays areshown in their nonenergized state. Boththe normally open and normally closed contacts of each relay areprovided with conventional contact filters. The filter usually includesa 30 ohm resistor in series with a 0.022 microfarad capacitor connectedfrom the common contact of the relay to both the normally open andnormally closed contacts. In this embodiment there are three commandinputs from computer 20 and two status outputs to computer 20. Theinputs are a resynchronization (RES) input coupled to the KRESl relay,an advance (ADV) input coupled to the KADVl relay, and a hold (HOL)input coupled by way of a normally closed contact of the KADVl relay toone side of the KHOLI relay. The other side of each of the relays KRESI,KADVl and KI-IOLl is coupled to a +40 volt power supply. The RES inputis enabled (grounded) when the computer wants to resynchronize theoperation of the local controller to a predetermined offset whenreturning the controller to local operation. The resynchronization mayrelate to interval 1 of the signal controller or more likely thesynchronization will relate to a system zero time reference point. Thismode of operation is discussed hereafter.

When the computer decides to control an intersection it waits to receivea status signal that indicates that the main street is about to turngreen. Then the HOL input goes to ground and the Kl-lOLl relay islatched. Relay KHOLZ is also latched, by way of thecircuit pathincluding resistor R1 and diode CR5. Capacitor C1 holds relay KHOLZlatched for about 2 seconds when power is interrupted to the relayKHOLZ. The contacts of relay Kl-lOL2 couple to an indicator light (notshown) of map display unit 23. As long as the relay KHOL2 is latchedthis indicator light would be illuminated. As long as the HOL ground isnot lifted for longer than two seconds the KHOLZ relay is latched andits normally open contact stays closed.

In another embodiment the HOL and ADV inputs may be implemented by asingle input signal. Thus, the KADVl relay would be omitted and theKl-lOLl relay would be latched to hold the intersection, and unlatchedto enable the controller at the intersection to advance.

The computer receives status information about the intersection from thecontacts of the main street green relay KMSGI and the check relay KCHK.When thestatus signals received from the slave unit indicate thatthemain street is about to turn green, the normally open contact ofrelay KMSGl closes thereby indicating to the computer that it can nowissue a HOL command to transfer operation from local to remote. Thestatus signals are sent over the telephone line 25 during both local andremote operation. However, during local operation (see FIG. 5A) it isnoted that line 25 is at ground potential except when status signals arebeing transmitted.

When the Kl-lOLl telay is latched by the HOL command its normally opencontact closes. This couples the tip telephone lead by way of thenormally open-contact of the KI-lOLl relay, resistor Rl4,.the normallyclosed contact of relay KRESI and resistor R6, to the +100 volt powersupply. FIG. 5B shows this positive voltage level that initiates theholding of the intersection. Once the intersection is held the computermay then advance the controller into interval (see FIG. 6) which is thefirst interval that illuminates the G1 lamp. This is accomplished byenergizing the KADVl relay for about 400 milliseconds, for example. TheKHOLI relay immediately drops out when the normally closed contact ofthe KADVI relay opens. The normally closed contact of the Kl-IOLI relaythen closes, connecting the tip telephone lead by way of re-.

sistor R5 to steering circuit 30. FIG. 5B shows this positive to groundtransition. The status signal is received during the 400 millisecondground interval of the advance command. The response of the slave unitto the HOL and ADV commands, and its generation of a status signal arediscussed later with reference to FIG. 4. For now it is sufficient tostate that the status signal has a duration of about 200 milliseconds,is negative during controller intervals 1 and 9-16, and is positiveduring controller intervals 2-8. FIGS. 5 and 6 indicate the telephoneline pattern and controller-intervals, respectively.

The response of the slave driver of FlGS. 3A and 33 to status signals isconsidered next. During both remote and local operation, each time thecontroller cam is advanced, a 200 millisecond polarized signal istransmitted over the telephone line. If the controller is in a phase Bgreen interval, for example, the S1 signal is false and the statusvoltage on the tip lead is positive for 200 milliseconds. If S1 is'truethe status voltage is negative, as shown in FIG. 5A. Thus, when thecontroller is not in main street green, and because the KADVI relay isstill in, the positive status voltage is transferred by way of thenormally closed contact of relay KHOLl and resistor R5 to steeringcircuit 30. Steering circuit 30 comprises diodes CR1-CR4 arranged in abridge circuit with resistors R2 and R3 and capacitors C2 and C3.Circuit 30 also includes zener diodes CR6 and CR7, status relays KSTA,and not-status relay KNST.

The ring lead of the telephone line is conventionally terminated toground through resistor R8. With a positive voltage on the tip lead,diodes CR2 and CR4 are forward biased and a path is provided throughrelay KNST and zener diode CR7. When the voltage increases sufficientlythe zener diode CR7 conducts and relay KNST latches. The back biaseddiodes CR1 and CR3 prevent relay KSTA2 from latching. Capacitor C3delays the dropout of relay KNST slightly when the positive voltage onthe tip lead terminates. This voltage across capacitor C3 is allowed tobleed off through resistor R3. Capacitor C3 and resistor R3 have arelatively long time constant. However, because the zener diode CR7 isin series with relay KNST, the voltage does not have to decrease thatmuch before KNST unlatches.

When the KNST relay is latched and the KSTA2 relay is unlatched, theirrespective contacts cause the not-main street green relay KNMG to belatched and. the main street relays KMSGI and KMSG2 to be unlatched. Thefirst positive status pulse latches KNST, and a ground is provided forthe KNMG relay through resistor R11 and the closed (normally open)contact of relay KNST. With the KSTA2 relay unlatched a positive levelis coupled via resistor R10 and the normally closed contact of relayKSTA2, to the other side of relay KNMG. When the KNST relay temporarilyfalls out after the status pulse ends, the normally open contact ofrelay KNMG, which was previously closed,- keeps the KNMG relay latched.Relay KNMG stays latched until a negative status pulse, (interval 9)indicating that the main street is about to turn green, is re-' ceivedby the slave driver.

In this embodiment, as long as relay KNMG stays latched the check relayKCHK is unlatched because the normally open contact of relay KSTA2 isopen. Also, the KMSGl'and KMSGZ relays are unlatched because thenormally closed contact of relay KNMG is open. The KMSGl contact whichcouples to the computer indicates that the intersection is not operatingin main street green. The KMSG2 contact which couples to the map displayunit 23 indicates the same condition. The KCI-IK contact also indicatesthat the controller is not in main street green otherwise check pulseswould be monitored as discussed hereafter.

As previously noted, when the interval just preceding the first mainstreet green interval is advanced to by the controller, the status pulseis negative on the tip lead. This 200 millisecond negative level forwardbiases diodes CR1 and CR3, assuming the relay KHOLI is unlatched whichis the case during local operation or during an advance pulse underremote operation. A path is then provided through these diodes CR1, CR3,relay KSTA2 and zener diode CR6. When the voltage increases sufficientlyzener diode CR6 conducts and relay KSTA2 latches. The back-biased diodesCR2 and I CR4 prevent relay KNST from latching. Capacitor C2 slightlydelays the dropout of relay KSTA2 when the negative voltage on the tiplead terminates after the 200 millisecond interval. The voltage acrosscapacitor C2 is allowed to bleed off through resistor R2. Capacitor C2and resistor R2 have a relatively long time constant. However, becausethe zener diode CR6 is in series with the relay KSTA2, the voltage doesnot have to decrease that much before KSTA2 unlatches.

When the KSTA2 relay is latched and the KNST relay is unlatched, theirrespective contacts cause relay KNMG to unlatch, and relays KCHK, KMSGland KMSG2 to latch. The first negative status pulse latches relay KSTA2,and its normally closed contact opens. The positive 100 volt level fedvia resistor R is then coupled by way of the closed (normally open)contact of relay KSTA2 and resistor R9 to one side of the KCHK relay,instead of to one side of the KNMG relay. The unlatching of relay KNMGcauses its normally closed contact to close thus providing a ground forone side of both relays KMSGI and KMSG2. The other side of relays KMSGland KMSG2 couple by way of resistors R12 and R13, respectively, to the+100 volt power supply. When the KSTA2 relay temporarily falls out afterthe termination of the negative status pulse, the 100 volt supply isapplied via resistor R10 to one side of relay KNMG. This relay, however,does not latch as no ground path is closed through the KNMG relay. Atthe same time the normally open contact of relay KSTA2 opens and powervia resistor R9 is interrupted to the KCI-IK relay. Capacitor C4prevents relay KCHK from falling out for about one second. Thus, thecontacts of the KCI'IK relay provide a check pulse coupled to thecomputer whenever a negative (main street green) status pulse isreceived over the tip telephone lead. Also, the normally open contact ofthe KMSGl relay closes upon the occurrence of the first negative statuspulse and stays closed until the main street green interval ends. Thecontacts of the KMSG2 relay connect to the map display unit 23 toindicate a main street green condition for the intersection.

FIG. 3C shows an alternate arrangement for the KMSGl, KMSG2, KNMG andKCI-IKl relays which provides a check pulse for cam advances both duringthe intervals when the main street is green and the intervals when it isnot green. In FIG. 3B the KCl-IKI relay was unlatched when not in mainstreet green and could not send a check pulse to the computer duringthose intervals. In FIG. BC the KCI-IKl relay may be latched andunlatched to send a pulse to the computer during main street green andmain street not green.

The arrangement of relays and corresponding contacts in FIG. 3C issimilar to the arrangement of FIG. 3B. The KMSGl and KMSG2 relays eachtie on one side to ground, and the other sides couple to resistors R12and R13 respectively. One side of relay KNMG couples via resistor R8 toits own normally open contact, and also via resistor R8 and diode D1 tothe normally open contact of the KNST relay. The other side of the KNMGrelay connects to the normally closed contact of the KSTA2 relay. Oneside of the KCI-IKl relay connects via diode D2 to ground and via diodeD3 to the normally open contact of the KSTA2 relay. The other side ofrelay KCHKl couples via resistor R10 to diode D4 and via diode D5 to thenormally open contact of the KNST relay.

The operation of the circuit of FIG. 3C is similar to that of FIG. 3B.When the system is not in a main street green interval, the KNST relay(see FIG. 3B) is latched for about 200 milliseconds, and the KSTA2 relayis unlatched. Thus, in FIG. 3C the KNMG relay is latched through theclosed (normally open) contact of the KNST relay, and the normallyclosed contact of the KSTA2 relay. The KMSGl and KMSG2 relays areunlatched because the normally closed contact of the KNMG relay is open(KNMG latched). A volt level is fed via the closed (normally open)contact of the KNST relay (KNST latched), diode D5, and resistor R10 toone side of the KCHKl relay, the other side of which is coupled viadiode D2 to ground, causing the KCl-IKI relay to temporarily latch. Whenthe status pulse terminates the KNST relay falls out, the +100 voltlevel is lifted from the KCIIKI relay, and eventually the KCHKI relayfalls out. Capacitor C4 and resistor R9 are tied in series between oneside of relay KCI-IKI and the commonly tied cathodes of diodes D4 andD5, and provide a decaying time constant that delays the unlatching ofrelay KCHKI. Thus, a check pulse is transmitted to the computer byvirtue of the latching and unlatching of the KCHKI relay during not mainstreet green. This indication is coupled to the computer via thecontacts of the KCHKI relay (see FIG. 3B).

The operation of the KCHKI relay during the main street green intervalsis substantially the same as discussed with reference to FIG. 3B. Duringthose intervals the KNST relay is unlatched and the KSTA2 relay issuccessively latched for about 200 milliseconds by the status pulse.When this occurs the normally open contact of the KSTA2 relay closes andthe normally open contact of the KNST relay opens. The KNMG relay fallsout due to the switching of the KSTA2 relay contacts. Thus a negative100 volt level is applied via diode D3 to one side of the KCl-IKl relay,the other side of which ties via resistor R10 and diode D4 to ground,latching relay KCI-IKI. When the status pulse terminates the KSTA2 relayfalls out, the l00 volt level is lifted from the KCHKl relay, andeventually the KCHKI relay falls out as determined by capacitor C4 andresistor R9. Therefore, a check pulse is also sent to the computer byvirtue of the latching and unlatching of the KCHKI relay during mainstreet green intervals. Many times it is the computer requirements thatdictate when check pulses are needed.

SLAVE UNIT A circuit implementation for slave unit 24 of FIG. 2 is shownin FIG. 4 with all the relays in their nonenergized state. Each relay isprovided with a conventional contact filter. Unit 24 connects overtwo-wire telephone line 15 to its corresponding slave driver 22, and hasconnections to the local signal controller to receive interval signalsand control advancement of the controller. In the non-actuated (nodetectors) system only one signal is coupled from the signal controllerto terminal 44 of the slave unit. This signal is referred to as the S1signal, and for an operational sequence as shown in FIG. 6, S1 is true(high) during intervals 1 and 9-16. In other embodiments S1 could betrue during different intevals depending upon whether the computerwanted to sense main street green or, for example, cross street green.FIG. 4 also shows the output terminal 42 which couples to the cam (notshown) of the signal controller. A signal is coupled from terminal 42 tothe signal controller to advance the cam during either local or computercontrolled operation. Another output signal is coupled at terminal 46 tothe signal controller during resynchronization operation. This mode ofoperation is used to periodically resynchronize the local controller tothe appropriate offset before changing from remote to local operation.This type of operation is discussed hereinafter.

There are five terminals V, W, X, Y and Z shown in FIG. 4 which connectin the following manner for nonactuated operation. Terminal V connectsto a conventional 60 cycle AC power supply. Terminals X and Y aredirectly connected while terminal Z is left open. An output from thedial of the controller connects to terminal W. During local operationthe dial controls cam advancement, and periodic AC signals are receivedat terminal W to cause such advancement. If the signal controller isoperating under local control, as discussed with reference to FIGS. 3Aand 3B, the KI-IOLI relay is not latched and the tip telephone lead isat ground except during the status pulse (see FIG. 5A). The step relay'KSTEl is unlatched except during a status'pulse and thus relays KRESZand KADVZ are not energized during local operation. The respectivecontacts of relays KRESZ and KADVZ are in the rest position shown inFIG. 4, and a path is provided from AC terminal V via resistor R and thenormally closed contacts of relays KRESZ and KADV2 to terminals X and Y,and the normally open contact of relay KI-IOL3. With relay KADVZunlatched, no power path is provided to relay KHOL3, and thus it remainsunlatched. The normally closed contact of relay KI-IOL3 stays closed andonly local cam advancement is enabled via resistor, R16 from terminal W.When a periodic AC signal is coupled to terminal W from the signalcontroller, this signal passes through resistor R17 to the gate of TRIACTRl causing it to conduct. One of the main terminals of TRIAC TRlconnects to a 60 cycle AC source, and the other connects to terminal 42.When TRIAC TRl is turned on by the gate voltage, AC is passed toterminal 42 and thence to the cam of the signal controller to cause itto advance. During local operation the advance signal is passed throughthe normally closed contact of relay KI-IOL3, while during remoteoperation the signal passes to TRIAC TRl via the normally open contactwhen closed by the latching of relay KHOL3.

When the system is operating under remote computer control, as discussed.withreference to FIGS. 3A and 3B the KI-IOLl relay is latched toholdthe intersection and unlatched to advance the intersection controller.The tip telephone lead is at a positive level during the hold and goesto ground for an advance. FIG. 5B shows the voltage pattern on thetelephone line for remote operation. Thus when the tip lead is positiveand the normally closed contact of the KSTEI relay is closed, a path isprovided via resistor R18 to the steering circuit 40 of the slave unit.Steering circuit 40 comprises diodes CRl0-CR13 arranged in a bridgecircuit with resistors R20 and R21 and capacitors C20 and C21. Circuitalso includes zener diodes CR14 and CR15, advance relay KADVZ, andresynch relay KRES2. The steering circuit 40 may be identical to circuit30 of FIG. 3B and operates as discussed previously. When the tiptelephone lead is positive (during hold) diodes CR11 and CR13 areforward-biased and the KADV2 is latched. The normally open contact ofthe KADVZ relay closes, and a path is provided from AC terminal Vthrough resistor R15, the closed (normally open) contact of relay KADV2,diode CR16 and resistor R22 to one side of relay KHOL3, the other sideof which is grounded. Relay KHOL3 almost immediately latches and itsnormally open contact closes thereby interrupting local operation viaterminal W. Resistor R23 connects between resistor R22 and capacitorC22, and is of a suitable value to cause relay KHOLS to quickly latch.With relay KI-IOL3 latched and capacitor C22 charged diode CR17 which isin parallel with resistor R23 becomes forward-biased. Capacitor C22keeps the KI-IOL3 relay latched for about two seconds after power isinterrupted to the KI-IOL3 relay.

At some time thereafter the computer issues an advance ADV command (seeFIG. 3A) and the voltage on the tip telephone lead goes to ground. About100 1 milliseconds thereafter the KADV2 relay drops out and its normallyclosed contact closes. This closure provides a circuit path from ACterminal V, through the normally closed contacts of relays KRESZ andKADVZ, to terminals X and Y, through the now closed (normally open)contacts of relay KHOL3 to TRIAC TRl. This causes an AC signal onterminal 42 to advance the controller cam under computer control. Theduration of this ground advance pulse as shown in FIG. 5B is about 400milliseconds.

When the KADV2 relay unlatches the AC power from terminal V is no longerfed via diode CR16 to the Kl-IOL3 relay. However, the charge uponcapacitor C22 holds relay KHOL3 latched for up to 2 seconds. When theadvance pulse is only 400 milliseconds the Kl-IOL3 relay does not fallout and the intersection is held under computer control. To returnthe'intersection to local control the hold (positive) level is dropped(ground) for over 2 seconds, relay KHOL3 falls out and its normallyclosed contact closes, returning the signal controller to local dialoperation by way of terminal W.

Thus, during local operation the cam is advanced from the dial, andduring remote operation it is advanced under computer control. In eithercase TRIAC TR'l conducts to transfer the AC voltage to terminal 42. WhenTRIAC TRl conducts, the AC signal coupled to terminal 42 also is fed viadiode CRIS, resistor R24, capacitor C24 and resistor R25 to one side ofthe KSTEl relay, the other side ofwhich is grounded. Diode CR18rectifies the AC signal and passes only the positive portion of thesignal. Diode CR19 prevents reverse current from passing through relayKSTEl by clamping the cathode of diode CR19 to essentially groundpotential. Without diode CR19, a reverse current would flow throughrelay KSTEl when capacitor C24 discharged. When capacitor C24 becomesfully charged current flow through relay KSTEl stops for the balance ofthe cam interval. The time constant including capacitor C24 andresistors R24 and R25 is de-' signed so that relay KSTEl is latched forabout 200 milliseconds, which is the duration of the status pulse asshown in FIG. 5B.

1 I tip lead, depending upon the state of the status relay KSTAl.

FIG. 4 shows a conventional rectifier circuit 47 for converting the ACsignal into a positive 100 volts at terminal 49 and a negative 100 voltsat terminal 51. The positive and negative voltages are coupled viaresistors R27 and R28 to the normally closed and normally open contactsof relay KSTAl, respectively. A positive voltage is applied to the tiptelephone lead when the KSTAI relay is pulled in while a negativevoltage is applied to the tip telephone lead when the KSTAl relay islatched. FIGS. 5 and 6 indicate that the S1 status signal is true duringintervals 1 and 9-16. Thus, when the controller advances to interval 9,under either local or remote control, the S1 signal impressed onterminal 44 goes high and the KSTAI relay latches. The circuit path isthrough rectifying diode CR20, resistors R26 and R27 and relay coilKSTAI to ground. The KSTAl relay stays latched during intervals 10-16and 1, in that order, because the S1 signal stays high. Capacitors C25and C26 tend to hold relay KSTAl latched if the S1 signal is temporarilyinterrupted. Thus, for the main street green intervals, relay KSTAl islatched and a negative status pulse of 200 milliseconds duration isimpressed on the tip telephone lead. Alternatively, for the otherintervals a positive status pulse is impressed on the tip telephone lead(see FIGS. 5A and 58). At the slave driver the step pulses operateeither relay KSTA2 or KNST depending on the pulse polarity, as discussedpreviously with reference to FIG. 3B.

SEMI-ACTUATED SYSTEM FIG. 7 is a block diagram similar to that shown inFIG. 2 for semi-actuated operation. The central station includescomputer 20, slave driver 22 and map display unit 23. The remote stationincludes slave unit 24, signal controller 26 and signal lights 28. Theslave driver 22 and slave unit 24 may use the circuitry shown in FIGS. 3and 4, respectively. FIG. 7 also includes a demand-respond unit 25 whichinterconnects to slave unit 24 and signal controller 26. One or moredetectors associated with phase B also couple to unit 25. FIG. 1 showsphase B detectors DB1 and DB2.

The system shown in FIG. 7 operates in a similar manner to thenon-actuated system of FIG. 2. However, the semi-actuated system isresponsive to the presence and absence of vehicles on phase B duringboth local and remote operation to control sequencing of the signalcontroller. During local operation the controller steps to a phase Arest or dwell position and remains in that position as long as novehicles are detected on phase B. After a predetermined interval thecomputer may want to control the intersection and advance the controllerout of phase A dwell regardless of whether any phase B detections haveoccurred. The computer issues a hold command which transfers operationto the remote mode. The computer then issues an advance command of 400milliseconds duration. If no phase B vehicles have been yet detected,the advance pulse has no effect on the signal controller and thecontroller does not advance out of phase A dwell. The controller willalways advance out of dwell, however, in either local or remoteoperation when a phase B detector is actuated. Also, when the recallswitch for remoteor local operation is active, the controller willadvance out of dwell.

If no phase B detections occur during remote operation the computer canissue a force-off command to force the controller to advance even thoughno phase B detections were received. This force-off command is issued byextending the typical advance pulse from 400 to 800 milliseconds. Thus,in FIG. 3 the ADV input from computer 20 would be present for 800milliseconds. This condition is sensed by demand-response unit 25 andthe controller advances.

In another embodiment of the invention a demandrespond unit is used forboth phase A and phase B. The second unit would have one or more phase Adetectors coupled to it and would keep the controller in a phase B dwellinterval until a call (vehicle or pedestrian detection) was receivedfrom the phase A detector.

DEMAND-RESPOND UNIT DWELL FIG. 8 shows one circuit implementation forthe demandrespond unit 25 shown in FIG. 7. This unit is responsive onthe phase A dwell interval to phase B detections. Hereafter, in FIG. 9,another demand-respond unit is considered. That unit is responsive onthe phase A dwell, for example, to phase B detections, and in additionis responsive to continued detections (extensions) on its own phase.

The diagram of FIG. 6 also applies to the demandrespond unit of FIG. 8.Thus, the S1 signal is still true during intervals 1 and 9-16. The S2(DWELL) signal from the controller is true only during interval 16,whereas the S3 (MASK) signal is true during intervals 10 and 16. The S4(MEMORY) signal is true during intervals 7-l6. The function of theseparticular signals is discussed with reference to FIG. 8.

The S2-S4 signals from the signal controller are coupled to terminals52-54, respectively, of the demandrespond unit. The S1 signal couples tothe slave unit 24 as previously discussed with reference to FIG. 4.Other external connections are made to the slave unit of FIG. 4. Theterminals V-Z of FIG. 4 connect directly to the like terminals V-Z ofFIG. 8. There is one other connection from the dial unit of thecontroller to terminal 55. In the non-actuated system the dial wasconnected directly to the slave unit.

During local operation the KHOL3 relay (see FIG. 4) is not latched, itsnormally open contact is open, and cam advancement can only occur by anAC signal from the controller dial. This signal is coupled to terminal55, and if the actuate relay KACT is unlatched, it passes by way of thenormally closed contact of relay KACT to terminal W, and thereaftercauses conduction of TRIAC TRl and advancement of the cam (see FIG. 4).The KACT relay is unlatched, permitting local cam advancement in allcontroller intervals except interval 16 which is the phase A DWELLinterval. Thus, when the controller advances under local operation tothe phase A DWELL interval, the S2 signal goes high enabling the KACTrelay to latch but only if no detection has previously occurred on phaseB. If a detection has occurred the KACT relay does not latch and thecontroller sequences out of the phase A DWELL interval through acomplete traffic cycle.

In FIG. 8 the memory relay KMRY is enabled by the S4 signal which is anAC signal from the controller applied to terminal 54 during intervals7-16 (see FIG. 6). This signal is rectified in the positive direction bydiode CR24 and passes by way of resistors R30 and R31 to one side ofrelay KMRY, Zener diode CR25 controls

1. A traffic control system comprising: a central control station; atleast one local station including a traffic signal controller forcontrolling the traffic intersection; a two wire communication lineintercoupling the central control station and the local station; saidcentral control station including circuit means for selectivelyimpressing a first signal level on the communication line when thecentral station is conditioned to control the local station and a secondsignal level on the communication line for a predetermined duration whenthe central station is conditioned to advance the signal controller atthe local station; said local station including circuit means responsiveto said first signal level for selectively rendering said local stationresponsive to said second signal level and means thereafter responsiveto said second signal level for advancing said signal controller; saidlocal station also including means responsive to the advance of saidsignal controller for generating a status signal during saidpredetermined duration of the second signal indicative of advancement ofthe controller; said central control station including means forreceiving said status signal over said two-wire communication lineduring said predetermined interval, wherein said local signal controlleris adapted to control traffic flow on at least two streets associatedwith the intersection at least one vehicle detector being associatedwith at least one of the streets, said local station comprisingdetection responsive means intercoupled with said vehicle detector, thecircuit means of the local station, and the local signal controller, andincluding a vehicle memory for storing a vehicle presence indicationfrom said vehicle detector.
 2. A traffic control system as set forth inclaim 1 wherein said central control station includes a computerprogrammed to transmit control commands to said circuit means of saidcentral control station.
 3. A traffic control system as set forth inclaim 1 wherein said detection responsive means comprises controlleradvance control means coupled to the means for advancing the signalcontroller and the vehicle memory for selectively inhibiting said meansfor advancing during at least a first interval of said signal controllerwhen no vehicles have been detected by said vehicle detector therebypreventing controller advance out of the first interval.
 4. A trafficcontrol system as set forth in claim 3 wherein said detection responsemeans also comprises timing circuit means coupled to said controlleradvance control means and adapted to receive said advance signal havinga predetermined duration; said timing circuit means responsive to saidadvance signal and the presence of a vehicle presence indication forpassing said advance signal to said means for advancing during the firstinterval.
 5. A traffic control system as set forth in claim 4 whereinsaid timing circuit means is also responsive to said advance signalwhich occurs during the absence of a vehicle detection for blocking saidadvance signal and preventing the signal controller from advancing outof the first interval.
 6. A traffic control system as set forth in claim4 wherein said timing circuit means is also responsive to an extendedadvance signal generated at said central control station and whichoccurs during the absence of a vehicle detection for passing at leastpart of said extended advance signal to said means for advancing.
 7. Atraffic control system as set forth in claim 4 wherein said timingcircuit means is also responsive during a vehicle extension intervalassociated with one phase of said signal controller to said advancesignal and the continued indication of detected vehicles associated withanother phase for blocking said advance signal and preventing the signalcontroller from advancing out of the extension interval.
 8. A trafficcontrol system as set forth in claim 7 wherein said timing circuit meansis also responsive to said advance signal which occurs during theabsence of detected vehicles over a predetermined time period forpassing said advance signal to said means for advancing during theextension interval.
 9. A traffic control system as set forth in claim 7wherein said timing circuit means is responsive to an extended advancesignal generated at said central control station during said extensioninterval associated with one phase and the continued indication ofvehicle detections associated with another phase for passing at leastpart of said extended advance signal to said means for advancing.
 10. Atraffic control system comprising: a central control station; at leastone local station including a traffic signal controller for controllingthe traffic intersection; a two-wire communication line intercouplingthe central control station and the local station; said central controlstation including circuit means for selectively impressing a firstsignal level on the communication line when the central station isconditioned to control the local station and a second signal level onthe communication line for a predetermined duration when the centralstation is conditioned to advance the signal controller at the localstation; said local station including circuit means responsive to saidfirst signal level for selectively rendering said local stationresponsive to said second signal level and means thereafter responsiveto said second signal level for advancing said signal controller; saidlocal station also including means responsive to the advance of saidsignal controller for generating a status signal during saidpredetermined duration of the second signal indicative of advancement ofthe controller; said central control station including means forreceiving said status signal over said two-wire communication lineduring said predetermined interval, wherein said local signal controlleris adapted to control traffic flow on a main street and a cross street,at least one vehicle detector being associated with the main and crossstreets respectively; said local station comprising at least twodetection responsive means, one coupled to the main street vehicledetector and the other coupled to the cross street detector, each saiddetection responsive means also coupled to the circuit means of thelocal station and the local signal controller and including a vehiclememory coupled to the vehicle detector for storing a vehicle demandindication.
 11. A traffic control system as set forth in claim 10wherein each said detection responsive means comprises controlleradvance control means coupled to the means for advancing and its vehiclememory, for selectively inhibiting said means for advancing during botha main street dwell interval of said signal controller when no vehicleshave been detected by the cross street vehicle detector, and a crossstreet dwell interval of said signal control when no vehicles have beendetected by the main street vehicle detectors, respectively.
 12. Atraffic control system as set forth in claim 11 wherein each of saidcontroller advance control means selectively inhibits said means foradvancing during both a main street extension interval when vehiclescontinue to be detected by the main street detector, and a cross streetextension interval when vehicles continue to be detected by the crossstreet detector, respectively.
 13. A traffic control system as set forthin claim 10 wHerein at least two vehicle detectors are associated withthe main street and the cross street respectively, and the local stationcomprises one detection responsive means coupled to each vehicledetector; said local station including AND circuit means intercouplingthe detection responsive means associated with one of the streets forpermitting advancement out of the extension interval associated withthat street only when vehicle indications associated with both vehicledetectors associated with that street terminate.
 14. A traffic controlsystem comprising: a central control station; at least one local stationincluding a traffic signal controller for controlling the trafficintersection; a two-wire communication line intercoupling the centralcontrol station and the local station; said central control stationincluding circuit means for selectively impressing a first signal levelon the communication line when the central station is conditioned tocontrol the local station and a second signal level on the communicationline for a predetermined duration when the central station isconditioned to advance the signal controller at the local station; saidlocal station including circuit means responsive to said first signallevel for selectively rendering said local station responsive to saidsecond signal level and means thereafter responsive to said secondsignal level for advancing said signal controller; said local stationalso including means responsive to the advance of said signal controllerfor generating a status signal during said predetermined duration of thesecond signal indicative of advancement of the controller; said centralcontrol station including means for receiving said status signal oversaid two-wire communication line during said predetermined interval,wherein said local station includes an enabling circuit coupled to saidmeans for advancing the signal controller and operative during at leasta second interval of operation of said signal controller during localcontrol of said signal controller to permit the first signal leveltransmitted from said central control station over said communicationline to control the local station thereby reverting the system fromlocal to remote central control.
 15. A traffic control system as setforth in claim 14 wherein said enabling circuit inhibits said firstsignal level during other intervals than said at least a second intervalduring local control of said signal controller thereby preventing thecentral station from controlling local operation.
 16. A traffic controlsystem comprising: a central control station; at least one local stationincluding a traffic signal controller for controlling the trafficintersection; a two-wire communication line intercoupling the centralcontrol station and the local station; said central control stationincluding circuit means for selectively impressing a first signal levelon the communication line when the central station is conditioned tocontrol the local station and a second signal level on the communicationline for a predetermined duration when the central station isconditioned to advance the signal controller at the local station; saidlocal station including circuit means responsive to said first signallevel for selectively rendering said local station responsive to saidsecond signal level and means thereafter responsive to said secondsignal level for advancing said signal controller; said local stationalso including means responsive to the advance of said signal controllerfor generating a status signal during said predetermined duration of thesecond signal indicative of advancement of the controller; said centralcontrol station including means for receiving said status signal oversaid two-wire communication line during said predetermined interval,said means for generating a status signal including means for generatinga status signal of a first polarity during a firsT predetermined periodand a status signal of a second polarity during a second successivepredetermined period.
 17. A traffic control system comprising: a centralcontrol station; at least one local station including a traffic signalcontroller for controlling the traffic intersection; a two-wirecommunication line intercoupling the central control station and thelocal station; said central control station including circuit means forselectively impressing a first signal level on the communication linewhen the central station is conditioned to control the local station anda second signal level on the communication line for a predeterminedduration when the central station is conditioned to advance the signalcontroller at the local station; said local station including circuitmeans responsive to said first signal level for selectively renderingsaid local station responsive to said second signal level and meansthereafter responsive to said second signal level for advancing saidsignal controller; said local station also including means responsive tothe advance of said signal controller for generating a status signalduring said predetermined duration of the second signal indicative ofadvancement of the controller; said central control station includingmeans for receiving said status signal over said two-wire communicationline during said predetermined interval, wherein the circuit means atsaid central control station is adapted to selectively impress a thirdsignal level on the communication line when the central control stationis conditioned to re-synchronize the timing at the local station.
 18. Atraffic control system as set forth in claim 1 wherein said localstation includes means for preventing local signal controlleradvancement during the pendency of the third signal level therebymaintaining the signal controller under remote control duringre-synchronization.